The subject matter disclosed herein relates generally to static starter systems and, more particularly, to systems and methods for determining the state of a solid state semiconductor device that may be used in such static starter systems.
In some gas turbine systems, a gas turbine may need to be started and accelerated to a desired speed profile on relatively short notice. The speed profile may be one that provides desired starting conditions for the gas turbine. In such applications, a static starter system may be provided in conjunction with an electrical generator acting as a synchronous motor, which may be operatively coupled to a shaft of the gas turbine. During the starting sequence of the gas turbine system, the static starter system may deliver a variable frequency current to drive and control the electrical generator (e.g., by modulating exciter field voltage and/or stator current), which in turn drives the main shaft of the gas turbine into rotation. Once the static starter system has accelerated the gas turbine to a desired speed (e.g., matching the desired speed profile, which may represent a self-sustaining speed), the static starter system may disengage and electrically disconnect from the generator as the gas turbine enters normal (e.g., self-sustaining) operation.
To provide the variable frequency current to the generator, static starter systems may include power conversion circuitry. For instance, the power conversion circuitry may include multiple strings of solid state semiconductor switches, such as thyristors, arranged within source and load bridges to produce a variable frequency AC output that is supplied to the generator. Control circuitry may provide gate firing signals to the solid state switches, which may transition to a conductive state when a gate pulse is received. During operation, error conditions may occur which may result in one or more solid state switches failing to conduct properly. Thus, in a string of series-connected solid state switches, the failure of one switch to conduct properly may result in the entire string not conducting. In existing static starter systems, it is difficult to detect and identify non-conducting solid state switches (e.g., using multimeters or other measuring instrumentation) during system operation, which may complicate troubleshooting and repair procedures. Disadvantageously, troubleshooting procedures in existing static starter systems may require an operator to power off the static starter system and to test each solid state switch using a trial and error technique to identify a non-conducting switch, which is not only inconvenient, but may undesirably result in equipment downtime.